A hypodermic needle is a hollow needle commonly used with a syringe to inject substances into the body. They may also be used to take liquid samples from the body, for example taking blood from a vein in venipuncture.
A hypodermic needle is used for instant delivery of a drug, or when the injected substance cannot be ingested, either because it would not be absorbed (as with insulin), or because it would harm the liver (as with testosterone). There are many possible routes for an injection.
Virtually all current hypodermic needles and their associated syringes are designed for single use because they are hard to decontaminate and need sharpening after repeat use. Re-using or sharing needles can transmit many blood-borne diseases, including HIV and hepatitis C. Needles are normally used only once and disposed of in a sharps container. This is has been true for decades in the industrialized West, but in developing countries, such as in most of Africa, needles are often reused—and sharpened and sterilized between uses. This means that such needles become a common route of infection, for AIDS and other diseases. The first non-African AIDS patient—and the first whose name is known—was Grethe Rask, a Danish surgeon in a rural hospital in Zaire in 1976. In this hospital, throwaway needles and gloves were a fantasy: needles and gloves were reused until obviously unusable. Although she was skilled in sterile procedure, she was often spashed with blood and other body fluids and may have accidentally suffered a needle-stick injury. Her story is told in Chapter 1 of And the Band Played On, by Randy Shilts.
Single-use syringes, which have automatic mechanisms to prevent reuse, have been developed to stop the spread of disease in medical settings, examples of which include medical-personnel exposure and immunizations  . One type is a pre-filled plastic blister with a hypodermic needle attached via a one-way valve. 
Hypodermic needles are normally made from a stainless-steel tube drawn through progressively smaller dies to make the needle. The end is bevelled to create a sharp pointed tip. This lets the needle easily penetrate the skin. When a hypodermic needle is inserted, the bevel should be facing upwards.
The diameter of the needle is indicated by the needle gauge. Various needle lengths are available for any given gauge. There are a number of systems for gauging needles, including the Stubs Needle Gauge, and the French Catheter Scale. Needles in common medical use range from 7 gauge (the largest) to 33 (the smallest) on the Stubs scale. Twenty-one-gauge needles are most commonly used for drawing blood for testing purposes, and sixteen- or seventeen-gauge needles are most commonly used for blood donation, as they are large enough to allow red blood cells to pass through the needle without rupturing (this also allows more blood to be collected in a shorter amount of time). Smaller-gauge needles will rupture the red blood cells, and if this occurs, the blood is useless for the patient that is receiving it. Although reusable needles remain useful for some scientific applications, disposable needles are far more common in medicine. Disposable needles are embedded in a plastic or aluminium hub that attaches to the syringe barrel by means of a press-fit (Luer) or twist-on (Luer-lock) fitting.
Hypodermic needles are usually used by medical professionals—physicians, nurses, paramedics—but they are sometimes used by patients themselves. This is most common with type one diabetics, who may require several insulin injections a day. It also occurs with patients who have asthma or other severe allergies. Such patients may need to take desensitization injections; or they may need to carry injectable medicines to use for first aid in case of a severe allergic reaction. Such patients often carry a syringe loaded with epinephrine; some also carry syringes loaded with Benadryl and Decadron. The rapid injection of these drugs may stop a severe allergic reaction, prevent anaphylactic shock, and make an emergency room trip unneccessary, although it may be disconcerting to spectators. Hypodermic needles are also used illicitly in recreational intravenous drug use, for this reason it is illegal in many jurisdictions to possess used or unused needles without an approved medical reason.
It is estimated that about 10% of the adult population may have a phobia of needles (Trypanophobia), and it is much more common in children. There is some basis for this phobia, because not all medical personnel have the necessary skill with hypodermic needles. This is particularly with intravenous uses, where incorrect insertion may easily damage the vein. It is not unusual for a nurse or doctor to take several tries to start an IV; a dozen tries is not unknown. Typically, a large hospital has one "deadeye"—usually, a pediatric anesthesiologist—who can put needles in tiny or contracted veins, but this person is not available 24/7.
- intravenous therapy
- needle biopsy
- Needle gauge comparison chart
- Needle remover
- Tuohy needle
- The Needle Phobia Information Site
- Needle Phobia and Dental Injections
- Drilled & Rolled End Surgical Suture Needles
- Hypodermic Productions: Because mediocrity is not excellence!